Solid carbon products comprising carbon nanotubes and methods of forming same

ABSTRACT

Methods of forming solid carbon products include disposing a plurality of nanotubes in a press, and applying heat to the plurality of carbon nanotubes to form the solid carbon product. Further processing may include sintering the solid carbon product to form a plurality of covalently bonded carbon nanotubes. The solid carbon product includes a plurality of voids between the carbon nanotubes having a median minimum dimension of less than about 100 nm. Some methods include compressing a material comprising carbon nanotubes, heating the compressed material in a non-reactive environment to form covalent bonds between adjacent carbon nanotubes to form a sintered solid carbon product, and cooling the sintered solid carbon product to a temperature at which carbon of the carbon nanotubes do not oxidize prior to removing the resulting solid carbon product for further processing, shipping, or use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/470,587, filed Mar. 27, 2017, which is a divisional of U.S. patentapplication Ser. No. 14/414,232, filed Jan. 12, 2015, now U.S. Pat. No.9,604,848, issued Mar. 28, 2017, which is a national phase entry under35 U.S.C. § 371 of International Patent Application PCT/US2013/049719,filed Jul. 9, 2013, designating the United States of America andpublished in English as International Patent Publication WO 2014/011631A1 on Jan. 16, 2014, which claims the benefit under Article 8 of thePatent Cooperation Treaty to U.S. Provisional Patent Application Ser.No. 61/671,022, filed Jul. 12, 2012, for “Solid Carbon ProductsComprising Carbon Nanotubes and Methods of Forming Same,” the disclosureof each of which is hereby incorporated herein in its entirety by thisreference.

TECHNICAL FIELD

Embodiments of the present disclosure relate to methods and systems forforming solid carbon products from carbon nanotubes including mixturesof various types of carbon nanotubes and mixtures of carbon nanotubeswith other substances.

BACKGROUND

The following documents, each published in the name of Dallas B. Noyes,disclose background information hereto, and each is hereby incorporatedherein in its entirety by this reference:

-   -   1. U.S. Patent Publication No. 2012/0034150 A1, published Feb.        9, 2012;    -   2. International Application No. PCT/US2013/000071, filed Mar.        15, 2013;    -   3. International Application No. PCT/US2013/000072, filed Mar.        15, 2013;    -   4. International Application No. PCT/US2013/000073, filed Mar.        15, 2013;    -   5. International Application No. PCT/US2013/000075, filed Mar.        15, 2013;    -   6. International Application No. PCT/US2013/000076, filed Mar.        15, 2013;    -   7. International Application No. PCT/US2013/000077, filed Mar.        15, 2013,    -   8. International Application No. PCT/US2013/000078, filed Mar.        15, 2013;    -   9. International Application No. PCT/US2013/000079, filed Mar.        15, 2013; and    -   10. International Application No. PCT/US2013/000081, filed Mar.        15, 2013.

Conventional methods of using CNTs (“carbon nanotubes”) in engineeringmaterials generally rely on embedding the CNTs in a matrix material.CNTs are currently processed in a wide variety of composite structuresusing metals, plastics, thermoset resins, epoxies, and other substancesas the matrix to hold the CNTs together, thus creating solid objects.The CNTs act as reinforcing material to improve properties of thematerials. Typical objectives of using carbon nanotubes in a matrix areto increase the strength, decrease weight, or to increase electrical andthermal conductivity of the composite.

Methods to make materials composed primarily of carbon nanotubes includespinning the carbon nanotubes into fibers and making “buckyrock.” U.S.Pat. No. 6,899,945, issued May 31, 2005, and entitled “Entangledsingle-wall carbon nanotube solid material and methods for making same”discloses a method for making buckyrock. Buckyrock is athree-dimensional, solid block material including an entangled networkof single-wall CNTs. Buckyrock is mechanically strong, tough, and impactresistant with a bulk density of about 0.72 g/cm³ (see Example 3 of U.S.Pat. No. 6,899,945). The single-wall CNTs in a buckyrock form arepresent in a random network. The random network of the CNTs appears tobe held in place by Van der Waals forces between the CNTs and byphysical entanglement or interference of the CNTs. One type of buckyrockis made by forming a slurry of CNTs in water, slowly removing water fromthe slurry to create a paste, and allowing the paste to dry very slowly,such that the CNT network of the paste is preserved during solventevaporation. Buckyrock can be used in various applications requiringlightweight material with mechanical strength, toughness, and impactresistance, such as ballistic protection systems.

Though conventional materials including CNTs have interesting and usefulproperties, the individual CNTs comprising these materials havesignificantly different properties. It would therefore beneficial toproduce materials having properties more comparable to the properties ofindividual CNTs.

BRIEF SUMMARY

Methods of forming solid carbon products include pressure compactionmethods such as extruding, die pressing, roller pressing, injectionmolding etc. to form solid shapes comprising a plurality of carbonnanotubes. The carbon nanotubes may optionally be mixed with othersubstances. Such solid shapes may be further processed by heating in aninert atmosphere to temperatures sufficient to sinter at least some ofthe CNTs so that covalent bonds form between adjacent CNTs. The methodsmay include forming a plurality of nanotubes, disposing the plurality ofnanotubes in a press, and applying heat and pressure to the plurality ofcarbon nanotubes to form the solid carbon product. When sintered, theresulting material is a novel composition of matter having two or moreCNTs with covalent bonding between them.

The solid carbon products, whether sintered or not, include interlockedCNTs that define a plurality of voids throughout the material. Thedimension of the interstitial voids may be controlled by a variety ofmethods including controlling the characteristic diameter of the CNTscomprising the solid carbon products, the inclusion of other materialsthat may create voids when removed from the solid carbon products, andthe pressure and temperatures at which the solid carbon products areformed.

Sintered solid carbon products include a plurality of covalently bondedcarbon nanotubes. Some methods include compressing a material comprisingcarbon nanotubes, heating the compressed material in a non-reactiveenvironment to form chemical bonds between adjacent carbon nanotubes andform a bonded carbon nanotube structure, and cooling the bonded carbonnanotube structure to a temperature at which carbon of the carbonnanotubes does not react with oxygen.

Other methods include first forming a solid carbon product bycompressing a material comprising carbon nanotubes and subsequentlyplacing the resulting solid carbon product into sintering conditions.The sintering conditions may include an inert environment, such as avacuum or inert atmosphere (e.g., argon or helium). The solid carbonproduct is heated to a desired temperature for a period of time toinduce covalent bonding between adjacent CNTs, after which the object iscooled below the oxidation temperature of carbon in air. The product maythen be removed from the sintering conditions.

Such methods may include any of a variety of standard industrialprocessing methods such as extrusion, die pressing, injection molding,isostatic pressing, and roll pressing. The sintering of the solid carbonproducts can be performed in a variety of apparatus such as are commonlyused in sintered powder metallurgy and sintered ceramic processing. Thesintering of the solid carbon products may include any of a variety ofmeans including induction heating, plasma arc discharge, hightemperature autoclaves and annealing furnaces, and other related devicesand methods as are known in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 4 are simplified illustrations of carbon nanotubes;

FIGS. 5 through 9 are simplified cross-sectional views of presses forforming solid carbon products;

FIGS. 10 and 11 are simplified illustrations depicting the structures oflinked carbon nanotubes; and

FIG. 12 is a graph showing bulk densities of solid carbon productsformed by compaction and sintering.

DETAILED DESCRIPTION

This disclosure includes methods of forming solid carbon products byapplying pressure to carbon nanotubes, and to methods for applying heatto the solid products formed by such processes. Solid carbon productsmay be useful in various applications, such as filters, reactors,electrical components (e.g., electrodes, wires, batteries), structures(e.g., beams, frames, pipes), fasteners, molded parts (e.g., gears,bushings, pistons, turbines, turbine blades, engine blocks), etc. Suchsolid carbon products may exhibit enhanced properties (e.g., strength,electrical or thermal conductivity, specific surface area, porosity,etc.) with respect to conventional materials. This disclosure includes anew class of materials that contain a plurality of CNTs formed intosolid shapes under pressure. When such solid shapes are sintered,covalent bonds form between at least some of the CNTs, forming solidshapes. This material has numerous useful properties.

As used herein, the term “sintering” means and includes annealing orpyrolizing CNTs at temperatures and pressures sufficient to inducecarbon-carbon covalent bonding between at least some of the adjacentCNTs between at least some of their contact points.

As used herein, the term “catalyst residual” means and includes anynon-carbon elements associated with the CNTs. Such non-carbon elementsmay include a nanoparticle of a metal catalyst in the growth tip of theCNTs, and metal atoms or groups of atoms randomly or otherwisedistributed throughout and on the surfaces of the CNTs.

As used herein, the term “green” means and includes any solid carbonproduct that has not been sintered.

CNTs may be created through any method known to the art, including arcdischarge, laser ablation, hydrocarbon pyrolysis, the Boudouardreaction, the Bosch reaction and related carbon oxide reductionreactions, or wet chemistry methods (e.g., the Diels-Alder reaction).The methods described herein are applicable to carbon nanotubesregardless of the method of manufacture or synthesis.

CNTs may occur as single-wall and multi-wall carbon nanotubes of variousdiameters ranging from a few nanometers to 100 nanometers in diameter ormore. CNTs may have a wide variety of lengths and morphologies, and mayoccur as substantially parallel “forests”, randomly tangled masses, or“pillows” of structured agglomerations. CNTs may also form or becompounded to form many different mixtures of CNTs with variouscombinations and distribution of the above characteristics (number ofwalls, diameters, lengths, morphology, orientation, etc.). Variousmixtures, when compounded and used to form the solid carbon productsdescribed herein, may result in products with specifically engineeredproperties. For example, the median void size of interstitial spacesbetween CNTs comprising solid carbon products typically is approximatelyproportional to the characteristic diameters of the CNTs used in formingthe solid carbon products. The median void size influences the overallporosity and density of the solid carbon products.

Various CNT features and configurations are illustrated in FIGS. 1through 4. FIG. 1 shows a single-wall CNT 100, in which carbon atoms 102are linked together in the shape of a single cylinder. The carbon atoms102 are covalently bonded into a hexagonal lattice, and thus form a CNT100 that appears as a single graphitic layer rolled into the form of atube. The CNT 100 may be conceptualized as a “rolled graphene sheet”lattice pattern oriented so that the carbon atoms 102 spiral at variousangles with regard to the axis of the CNT 100. The angle is called the“chirality” and common named forms include armchair and zigzag, asdescribed in Mildred S. Dresselhaus & Phaedon Avouris, Introduction toCarbon Materials Research, in Carbon Nanotubes: Synthesis, Structure,Properties, and Applications, 1, 6 (Mildred S. Dresselhaus, GeneDresselhaus, & Phaedon Avouris, eds., 2001), the entire contents ofwhich are incorporated herein by this reference. Many chiralities arepossible; CNTs 100 with different chiralities may exhibit differentproperties (e.g., CNTs 100 may have either semiconductor or metallicelectrical properties).

The CNT 100 has an inside diameter related to the number of carbon atoms102 in a circumferential cross section. The CNT 100 depicted in FIG. 1has a zigzag pattern, as shown at the end of the CNT 100. The diametermay also affect properties of the CNT 100. Single-walled CNTs 100 canhave many different diameters, such as from approximately 1.0 nm(nanometer) to 10 nm or more. A CNT 100 may have a length from about 10nm to about 1 μm (micron), such as from about 20 nm to about 500 nm orfrom about 50 nm to about 100 nm. CNTs 100 typically have an aspectratio (i.e., a ratio of the length of the CNT to the diameter of theCNT) of about 100:1 to 1000:1 or greater.

CNTs having more than one wall are called multi-wall CNTs. FIG. 2schematically depicts a multi-wall CNT 120 having multiple graphiticlayers 122, 124, 126, 128 arranged generally concentrically about acommon axis. Double-walled and triple-walled carbon nanotubes areoccasionally described as distinct classes; however, they may beconsidered as the smallest categories of multi-walled CNTs 120.Diameters of multi-wall CNTs 120 can range from approximately 3 nm towell over 100 nm. Multi-wall CNTs 120 having outside diameters of about40 nm or more are sometimes referred to as carbon nanofibers in the art.

FIG. 3 depicts two forms of multi-wall CNTs 140, 150. In the CNT 140,one single-wall CNT 142 is disposed within a larger diameter singe-wallCNT 144, which may in turn be disposed within another even largerdiameter single-wall CNT 146. This CNT 140 is similar to the CNT 120shown in FIG. 2, but includes three single-wall CNTs 142, 144, 146instead of four. Another form of multi-wall CNTs shown in FIG. 3 is CNT150, which may be conceptualized as a single graphene sheet 152 rolledinto tubes.

FIG. 4 schematically depicts a single-wall CNT 180 with an attachednanobud 182. The nanobud 182 has a structure similar to a sphericalbuckminsterfullerene (“buckyball”), and is bonded to the single-wall CNT180 by carbon-carbon bonds. As suggested by the structure shown in FIG.4, modifications may be made to the wall of a single-wall CNT 180 or tothe outer wall of a multi-wall CNT. At the point of bonding between thenanobud 182 and the CNT 180, carbon double-bonds can break and result in“holes” in the wall of the CNT 180. These holes may affect themechanical and electrical properties of the CNT 180. In single-wallCNTs, these holes may introduce a relative weakness when comparedunmodified cylindrical CNTs. In multi-wall CNTs, the outer wall may beaffected, but any inner walls likely remain intact.

Carbon nanotubes are typically formed in such a way that a nanoparticleof catalyst is embedded in the growth tip of the carbon nanotube. Thiscatalyst may optionally be removed by mild washing (e.g., by an acidwash). Without being bound to a particular theory, it is believed thatif the catalyst is left in place, catalyst atoms become mobilized duringthe sintering process, and may migrate to the surface or within thepores of the carbon nanotubes. This process may disperse the catalystatoms randomly, uniformly, or otherwise throughout the solid carbonproduct mass and may have a significant influence on the properties ofthe solid carbon product. For example, catalyst material may affectelectrical conductivity or the ability to catalyze other chemicalreactions.

The catalyst particles may be selected to catalyze other reactions inaddition to the formation of solid carbon. Catalyst particles may be anymaterial, such as a transition metal or any compound or alloy thereof.For example, catalyst particles may include nickel, vanadium oxide,palladium, platinum, gold, ruthenium, rhodium, iridium, etc. Because thecatalyst particles are attached to or otherwise associated with CNTs,each catalyst particle may be physically separated from other catalystparticles. Thus, the catalyst particles may collectively have a muchhigher surface area than a bulk material having the same mass ofcatalyst. Catalyst particles attached to CNTs may therefore beparticularly beneficial for decreasing the amount of catalyst materialneeded to catalyze a reaction and reducing the cost of catalysts.Compressed solid carbon products used as catalysts may, in manyapplications, benefit from the catalytic activity of both the CNT andthe metal catalyst particles embedded in the growth tip of the CNTs.

The CNTs used in the processes herein may be single-wall CNTs,multi-wall CNTs, or combinations thereof, including bi-modally sizedcombinations of CNTs, mixtures of single-wall and multi-wall CNTs,mixtures of various sizes of single-wall CNTs, mixtures of various sizesof multi-wall CNTs, etc. The CNTs may be in forms such as a sheet-moldedcompound, a pressure-molded compound, or as a pourable liquid. The CNTsmay be disposed within a press any other device structured andconfigured to provide pressure to the material. The press may include anextrusion die, a mold, a cavity, etc.

For example, in the press 200 shown in FIG. 5, CNTs 202 may be placed ina hopper 204 configured to feed material through an extrusion die 206.The press 200 includes an extrusion barrel 208 with a screw mechanism210 connected to a drive motor 212 to carry the CNTs 202 through theextrusion barrel 208 to the extrusion die 206. The extrusion barrel 208may optionally include means for heating the CNTs 202 as the CNTs 202pass through the extrusion barrel 208. The extrusion die 206 has anopening with a shape corresponding to the cross-sectional shape of apart to be formed in the press 200. Extrusion dies 206 may beinterchangeable, depending on the shape of objects desired. Somepossible shapes of extrusion dies 206 a, 206 b, 206 c are shown. Forexample, the extrusion die 206 may have an opening shaped like a circle,a regular polygon, an irregular polygon, an I-beam, etc. Extrusion dies206 can be structured to create objects of extruded CNTs of a variety ofshapes and sizes: symmetrical or asymmetrical, small to large. The CNTs202 may optionally be mixed with another material before or within thepress 200.

In some embodiments and as shown in the press 220 of FIG. 6, the CNTs202 are placed into a hopper 224 configured to feed material to a mold226. The press 220 includes a barrel 228 with a screw mechanism 230connected to a drive motor 232 to carry the CNTs 202 through the barrel228 to the mold 226. The barrel 228 may optionally include means forheating the CNTs 202 as the CNTs 202 pass through the barrel 228. Themold 226 has an opening with an interior shape corresponding to theexterior shape of a part to be formed in the press 220. Molds 226 may beinterchangeable, depending on the shape of objects desired. Somepossible shapes of molds 226 a and 226 b are shown. For example, themold 226 may have a shape of a screw or a propeller. The CNTs 202 mayoptionally be mixed with another material before or within the press 200to improve flowability, mold release, or other process properties. Suchmaterials may be subsequently removed by suitable means such as etching,pyrolysis, evaporation, etc. The resulting solid carbon product maysubstantially free of the additional material, and may includeessentially carbon and, in some embodiments, residual catalyst material.

In other embodiments and as shown in the press 240 of FIG. 7, the CNTs202 are placed into a body 244 having an interior shape defining anexterior of a product to be formed. The CNTs 202 may be placed betweentwo pistons 246, 248 surrounded by the body 244. The body 244 may havewalls 250 defining an interior cavity and configured to allow thepistons 246, 248 to slide freely. In other embodiments, a single pistonmay be configured to press CNTs against a body.

In an embodiment as shown in the press 260 of FIG. 8, CNTs 202 areplaced within a mold portion 262 having one or more surfacescorresponding to a shape of a product to be formed. One or moreadditional mold portions 264 are configured to press the CNTs 202against the mold portion 262, when pressed by pistons 266, 268, as shownin FIG. 9. Together, the mold portions 262, 264 define the shape of theproduct to be formed.

Pressure is applied to form the CNTs into a cohesive “green” body. Forexample, the screw mechanisms 210, 230 shown in FIGS. 5 and 6 applypressure to the CNTs 202 as the CNTs 202 pass through the presses 200,220. Extrusion through a die 206 as shown in FIG. 5 may be continuous(theoretically producing an infinitely long product) or semi-continuous(producing many pieces). Examples of extruded material include wire,tubing, structural shapes, etc. Molding, as in the press 220 shown inFIG. 6, is the process of manufacturing by shaping pliable raw material(e.g., CNTs 202) using a rigid pattern (the mold 226). The CNTs 202adopt the shape of the mold.

The pistons 266, 268 shown in FIGS. 8 and 9 are pressed toward the CNTs202, forming the CNTs 202 into a green body 270. The resulting greenbody 270 formed may be held together by relatively weak forces, suchthat the green body 270 may easily be further shaped (e.g., machined,drilled, etc.), but still holds its shape when handled. The CNTs of thegreen body 270 may each be in physical contact with one or more otherCNTs.

Heat is applied to green bodies to link the CNTs together into a morecohesive body in which at least some of the adjacent CNTs form covalentbonds between one another. For example, the CNTs may be heated at aheating rate from about 1° C./min to about 50° C./min to a temperatureof at least 1500° C., 1800° C., 2100° C., 2400° C., 2500° C., 2700° C.or even to just below the sublimation temperature of carbon(approximately 3600° C.). Pressure may also be applied concurrentlywith, before, or after heat is applied. For example, the CNTs may bepressed at 10 to 1000 MPa, such as 30 MPa, 60 MPa, 250 MPa, 500 MPa, or750 MPa. The green bodies may be subjected to a heated inertenvironment, such as helium or argon, in an annealing furnace. SinteringCNTs (i.e., subjecting them to heat in an oxygen-free environment)apparently creates covalent bonds between the CNTs at points of contact.The sintering of the CNTs typically occurs in a non-oxidizingenvironment, such as a vacuum or inert atmosphere so that the carbonnanotubes are not oxidized during the sintering. Sintering CNTs toinduce chemical bonding at the contact surfaces may improve desirablematerial properties such as strength, toughness, impact resistance,electrical conductivity, or thermal conductivity in the solid structureproduct when compared to the green material. The CNTs may also besintered in the presence of additional constituents such as metals orceramics to form composite structures, lubricants to aid processing, orbinders (e.g., water, ethanol, polyvinyl alcohol, coal, tar pitch etc.).Materials may be introduced as powders, shavings, liquids, etc. Suitablemetals may include, for example, iron, aluminum, titanium, antimony,Babbitt metals, etc. Suitable ceramics may include materials such asoxides (e.g., alumina, beryllia, ceria, zirconia, etc.), carbides,boride, nitrides, silicides, etc. In embodiments in which materialsother than CNTs are present, covalent bonding occurs between at leastsome of the CNTs, and the additional materials may become locked into amatrix of CNTs.

The CNTs in the sintered body have chemical bonds connecting oneanother. Chemical bonds, which are generally stronger than physicalbonds, impart different properties on the collection of CNTs thanphysical bonds. That is, the sintered body may have higher strength,thermal conductivity, electrical conductivity, or other properties thanthe green body from which it was formed.

When single-wall CNTs are covalently bonded to adjacent single-wallCNTs, holes can form on the surface of the CNTs as some of thecarbon-carbon bonds break, thus modifying the mechanical and electricalproperties of each single-wall CNT. The sintered single-wall CNTs,however, may still typically exceed non-sintered single-wall CNTs insuch properties as strength, toughness, impact resistance, electricalconductivity, and thermal conductivity. With multi-wall CNTs, typicallyonly the wall of the outer tube is modified; the internal walls remainintact. Thus, using multi-walled and bi-modally sized CNTs in, forexample, extrusion and molding processes, may yield solid structureswith properties that, in many respects, exceed practical limitations ofsingle-walled CNTs.

Sintering appears to cause covalent bonds to form between the walls ofCNTs at their contact points. That is, any given CNT may “cross-link”with an adjacent CNT at the physical point of contact of the two CNTs.Any given CNT having undergone sintering may be covalently bound tonumerous other CNTs (both single-wall CNTs and multi-wall CNTs). Thisincreases the strength of the resulting structure because the CNTs donot slide or slip at the bonding points. Unsintered, CNTs (e.g., inbuckyrock) may slide with respect to each other. Because the covalentbonding caused by sintering may occur at numerous sites in the mass ofCNTs, the sintered body has significantly increased strength, toughness,impact resistance, and conductivity over convention agglomerations ofCNTs.

FIG. 10 schematically depicts the cross-linked structure of twocovalently bound CNTs 280, 282 produced by sintering. When sintered,CNTs 280, 282 covalently bond at their contact points 284. Each CNT mayform covalent bonds with some or all of the other CNTs with which it isin contact during sintering. Due to the internal layering in amulti-wall CNT, covalent boding between the individual walls of themulti-wall CNT is likely to occur under sintering conditions. However,this covalent bonding has not yet been confirmed in testing. The heatingand optional pressurization of the CNTs in a sintering process aremaintained until the desired level of cross-linking has occurred. Thesintered CNTs are then cooled to a temperature at which the CNTs willnot spontaneously react with oxygen. Thereafter, the mixture may beexposed to air for further processing, storage, packaging, shipment,sale, etc.

In another embodiment, a CNT mixture is heated in a reactive environment(e.g., in the presence of oxygen, hydrogen, a hydrocarbon, and/oranother material). In this embodiment, heat and pressure are maintainedas needed until the reactants in the reactive environment have reactedwith one another or with the CNTs. The product is then cooled. In such aprocess, the reactants may form additional holes or pores in the CNTs,increasing the specific surface area of the sintered body.Alternatively, the reactants may deposit materials on the surface of theCNTs without affecting the underlying CNT structure.

In another embodiment, the CNT mixture is initially heated and sinteredin a nonreactive environment (e.g., in a vacuum, in the presence ofhelium, or in the presence of argon). Subsequent to sintering, the heatand pressure are changed to suitable reaction conditions and reactantsare added to the environment. Such reactants may include a variety ofmetals (as liquid or vapor), metal carbonyls, silanes, or hydrocarbons.The reaction of the reactants with one another or with the carbon of theCNT may fill some or all of the interstices of the CNT lattice withproducts of the reactions. Such processing with additional reactants mayin some cases be conducted during sintering, but may also be performedseparately. The heat and pressure are maintained until the desired levelof reaction (both cross-linking within the CNTs, and the reactionbetween the CNTs and the reactant) has occurred. The reacted mixture isthen cooled and removed from the reaction environment for furtherprocessing, storage, packaging, shipment, sale, etc.

FIG. 11 schematically depicts a mass 300 of covalently bound CNTs 302.The CNTs 302 bind through sintering with other CNTs 302 (multi-wall orsingle-wall CNTs) through mutual contact points 304, binding theaggregate together into a highly cross-linked structure. The resultantbinding may create a material of significant strength, toughness, impactresistance, and electrical and thermal conductivity.

During the sintering process, the green body may shrink, correspondingwith a decrease in the size of voids among the CNTs. However, thesintered body may remain porous due to the porosity of each CNT (i.e.,the center of the CNT) and due to voids between and among CNTs. Thesintered body may have pores or voids having a median minimum dimensionof less than about 1 μm, less than about 500 nm, less than about 100 nm,less than about 50 nm, or even less than about 10 nm. That is, each voidmay have two or more dimensions (e.g., a length, a width, and a height,each perpendicular to the others, or a diameter and a length), measuredin different directions. The voids need not be regularly shaped. The“minimum dimension” is defined as the minimum of the two or moredimensions of a single void. The “median minimum dimension” is definedas the median of these minimum dimensions for a group of voids.

A sintered body as described herein may have a high specific surfacearea, due to voids between CNTs and within CNTs (i.e., because the CNTsare hollow). For example, a sintered body may have a specific surfacearea of at least about 100 m²/g, at least about 500 m²/g, at least about750 m²/g, at least about 900 m²/g, or even at least about 1000 m²/g. Thespecific surface area can be controlled by the characteristic diametersor mixture of diameters of the CNTs used in forming the solid carbonproduct. For example, small-diameter single-wall CNTs have specificsurface areas up to approximately 3000 m²/g, while large-diametermulti-wall CNTs have specific surface areas of approximately 100 m²/g.

A sintered body may have a high electrical conductivity. For example, asintered body may have an electrical conductivity of at least about1×10⁵ S/m (Siemens per meter), at least about 1×10⁶ S/m, at least about1×10⁷ S/m, or even at least about 1×10⁸ S/m. The electrical conductivitycan be controlled by the types of CNTs used, the chirality of the CNTsused, the sintering conditions, and the quantity of resulting covalentbonds in the solid carbon product. For example, single-wall CNTs with ametallic chirality have a much higher electrical conductivity thanmulti-wall CNTs. As a further example, an increase in the number ofcovalent bonds appears to correlate with an increase in conductivity.

A sintered body may also have a high thermal conductivity. For example,a sintered body may have a thermal conductivity of at least about 400W/m·K (watts per meter per Kelvin), at least about 1000 W/m·K, at leastabout 2000 W/m·K, or even at least about 4000 W/m·K. The thermalconductivity of the resulting solid carbon product may be controlled bythe types of CNTs used and the chirality of CNTs used. For example,single-wall CNTs with a metallic chirality have much high thermalconductivity than large multi-wall CNTs.

CNTs may alternatively be pressed after the sintering process by, forexample, extrusion or molding, as described above with respect to FIGS.5 through 9. In some embodiments, the sintering process may be part ofthe formation of the desired object. For example, a section of theextrusion barrel may heat the CNTs to the sintering temperature in aninert atmosphere for an appropriate amount of time to cause sintering.Such heating may be, for example, induction heating or plasma archeating. Thus, sintered CNTs may be extruded. The sintered CNTs mayoptionally be mixed with another material such as a metal, a ceramic, orglass. The material may be pressed or pulled through a die under eitherextreme heat or cold. The material, forced into a given shape, is heldin place for a period of time and at sintering temperatures andpressures, and then returned to normal atmospheric conditions. Theproducts may be continuous, such as wires, or may be discrete pieces,such as bolts, propellers, gears, etc. Molding of sintered or sinteringCNTs typically involves either using the CNT material in concentratedform (i.e., with minimal impurities) or in forming a moldable compositewith another material, such as a metal. The moldable material is placedor poured into a rigid mold, held at a particular temperature andpressure, and then cooled back to normal atmospheric conditions.

In some embodiments, an incremental manufacturing method may be employedwherein CNTs (either compressed or not) are placed in a nonreactiveenvironment, such as in an inert gas autoclave. The CNTs are sintered toform covalent bonds between the CNTs in the surface layer and theunderlying layer. For example, a laser may irradiate a portion of theCNTs in a pattern. Additional CNTs are deposited over the sintered CNTs,and in turn sintered. The sintering process is repeated as many times asnecessary to achieve a selected thickness of sintered CNTs. The sinteredCNTs are then cooled to a temperature below which the CNTs do not reactwith oxygen or other atmospheric gases. The sintered CNTs may then beremoved from the nonreactive environment without contaminating thesintered CNTs. In some embodiments, the sintered CNTs are cooled andremoved from the nonreactive environment before deposition of eachadditional portion of CNTs.

In certain embodiments, sintered solid carbon products are formed in abelt-casting operation. A layer of CNTs is placed on a moveable belt.The belt moves the CNTs into a chamber containing a nonreactiveenvironment. The CNTs are sintered in the chamber, then cooled (e.g., ina portion of the chamber), and removed from the chamber. The process maybe operated continuously, such as to form a sheet of sintered CNTs.

In some embodiments, solid carbon products are further treated byelectrodeposition to fill interstices in the solid carbon products withanother material. A solution having materials to be deposited isprepared. The solvent of the solution may be water, an organic solvent,or an inorganic solvent. The solute may include a material such as ametal salt, an organic salt, a metalorganic salt, etc. Electroplatingsolutions are known in the art and not described in detail herein. Thesolid carbon product to be treated is contacted with the solution, suchas by immersing the body in the solution. An electric potential (adirect-current voltage or an alternating-current voltage) is applied tothe body to induce electrodeposition of one or more components of thesolution. The composition, potential, temperature, and/or pressure aremaintained until a selected amount of the material is deposited onto thesolid carbon product. The solid carbon product is then removed from thesolution and rinsed to remove excess solution.

Solid carbon products formed as described herein each include aplurality of cross-linked CNTs. The CNTs define a plurality of voids,which may have a median minimum dimension of less than about 1 μm, lessthan about 500 nm, less than about 100 nm, less than about 50 nm, oreven less than about 10 nm. Some or all of the CNTs may include a metal,such as a metal particle from which the CNTs were formed, or a metalcoating on the CNTs. The solid carbon products may be structural members(e.g., beams), fasteners (e.g., screws), moving parts (e.g., propellers,crankshafts, etc.), electrically conductive members (e.g., electrodes,wires, etc.), or any other form. The solid carbon product may includeanother material dispersed in a continuous matrix surrounding and incontact with the CNTs. The solid carbon products may have improvedstrength, toughness, impact resistance, and electrical and thermalconductivity in comparison to conventional materials.

In some embodiments, the solid carbon products also include othermorphologies of carbon, interspersed with or otherwise secured to theCNTs. For example, buckyballs may be connected to some of the CNTs. Asanother example, one or more graphene sheets may be formed over all or aportion of a solid carbon product.

Both the compressed solid carbon products and the sintered solid carbonproducts described herein have a wide variety of potentially usefulapplications. For example, the compressed solid carbon products may beused as filters, molecular sieves, catalysts, and electrodes inapplications where the additional mechanical integrity achieved throughsintering is not necessary. The sintered solid carbon products can beused in the applications in which compressed solid carbon products canbe used and in a wide variety of additional applications requiringadditional mechanical integrity, electrical properties, and othermaterial-property enhancements achieved through sintering.

Sintered solid carbon products may be useful components of armor becauseof their mechanical integrity, ability to absorb compressive loads witha high spring constant, and ability to dissipate heat. That is, sinteredsolid carbon products may be used to form projectile-resistantmaterials, such as armor plates, bullet-proof vests, etc. The lightweight of the solid carbon products could improve mission payloads,increase vehicle range, and alter the center of gravity. For example,armor materials including sintered solid carbon products may bebeneficial in preventing injury and death of occupants of vehicles suchas Mine Resistant Ambush Protected vehicles (“MRAPs”), which are proneto tipping. Sintered solid carbon products as described herein may beeffective in light-weight armament systems such as mortar tubes, gunbarrels, cannon barrels, and other components. Sintered solid carbonproducts may also be beneficial in aerial vehicles, such as aircraft,spacecraft, missiles, etc.

EXAMPLES Example 1: Sintering of Compacted CNTs

CNTs were formed as described in U.S. Patent Publication No.2012/0034150 A1. Samples of approximately 1.0 grams to 1.25 grams ofCNTs each were pressed in 15-mm diameter dies using a 100-ton (890-kN)press. The pressed samples were placed in an inert gas furnace (Model1000-3060-FP20, available from Thermal Technology, LLC, of Santa Rosa,Calif.) and heated under vacuum at a rate of 25° C. until the samplesreached 400° C. This temperature was maintained for 30 minutes to allowthe samples to outgas any oxygen, water, or other materials present. Thefurnace was then filled with inert gas (argon or helium) at 3-5 psi (21to 34 kPa) above atmospheric pressure. The furnace was heated at a rateof 20° C./min until the sample reached 1500° C. This temperature wasmaintained for 30 minutes. Heating continued at 5° C./min to a sinteringtemperature, which was maintained for a dwell time of 60 minutes. Thesamples were then cooled at 50° C./min to 1000° C., after which thefurnace was shut down until the samples reached ambient temperature. Thesample masses, compaction pressures, and sintering temperatures for thesamples are shown in Table 1 below. The inert gas was helium for thesamples sintered at 2400° C. and was argon for the other samples.

TABLE 1 Samples prepared in Example 1 Compaction Sintering Sample Mass(g) Pressure (MPa) Temperature (° C.) 1 1.076 500 1800 2 1.225 750 18003 1.176 250 1800 4 1.113 500 2100 5 1.107 750 2100 6 1.147 250 2100 71.103 500 2400 8 1.198 750 2400 9 1.121 250 2400 10 1.128 250 1900 111.209 500 1900 12 1.212 750 1900 13 1.101 250 2000 14 1.091 500 2000 151.225 750 2000 16 1.078 250 1700 17 1.179 500 1700 18 1.157 750 1700

Samples 1 through 18 were harder and more robust than were the samplesbefore the heating process. At the highest sintering temperature of2400° C. (samples 7 through 9), the sintered pellets are flakier thanthe other sintered samples. All the samples prepared in Example 1 werequalitatively observed to be hard.

Pycnometry tests show that the skeletal density decreases from 2.2 g/cm³for raw powders and raw compactions to 2.1 g/cm³, 2.08 g/cm³, and 2.05g/cm³ for the samples sintered at 1800° C., 2100° C., and 2400° C.,respectively. Bulk density also decreased after sintering, in almostevery case to less than 1.0 g/cm³. Pellet thickness increased 5% to 9%during sintering, with the higher pressure compactions expanding morethan the lower pressure compactions. The bulk densities of Samples 1through 9 are shown in Table 2 and in FIG. 12.

TABLE 2 Properties of samples prepared in Example 1: After CompactionAfter Sintering Compaction Skeletal Bulk Sintering Skeletal Bulk Sam-Pressure Density Density Temperature Density Density ple (MPa) (g/cc)(g/cc) (° C.) (g/cc) (g/cc) 1 600 2.1992 1.043 1800 2.1095 0.960 2 9002.2090 1.095 1800 2.0993 0.994 3 300 0.990 1800 2.1131 0.921 4 600 1.0632100 2.0680 0.971 5 900 1.084 2100 2.0817 0.992 6 300 0.999 2100 2.08290.910 7 300 0.985 2400 2.0553 0.932 8 600 1.069 2400 2.0479 1.009 9 9001.102 2400 2.0666 0.991

Example 2: Spark Plasma Sintering of CNTs

CNTs were formed as described in U.S. Patent Publication No.2012/0034150 A1. Graphite foil (available from Mineral Seal Corp., ofTucson, Ariz.) was lined into 20-mm diameter dies, and 2.0 g to 4.0 g ofCNTs were placed over the foil. The samples were placed in a sparkplasma sintering (SPS) system (model SPS 25-10, available from ThermalTechnology, LLC, of Santa Rosa, Calif.). An axial pressure ofapproximately 5 MPa was applied to the CNT samples, and the SPS systemwas then evacuated to less than 3 mTorr (0.4 Pa). The sample was heatedat 150° C./min to 650° C., and this temperature was maintained for oneminute to allow the vacuum pump to re-evacuate any materials out-gassed.The pressure was increased to the compaction pressure of 30 MPa or 57MPa, while simultaneously increasing the temperature at a rate of 50°C./min to 1500° C. The temperature and pressure were maintained for oneminute. The temperature was then increased at 50° C./min to thesintering temperature, and maintained for 10 min or 20 min. After thedwell, the pressure was reduced to 5 MPa, and the sample allowed to coolat 150° C./min to 1000° C., after which the furnace was shut off untilthe samples reached ambient temperature.

The sample masses, compaction pressures, compaction rates, sinteringtemperatures, and dwell times for the samples are shown in Table 2below.

TABLE 3 Samples prepared in Example 2: Compaction Compaction SinteringDwell Mass Pressure rate Temperature time Sample (g) (MPa) (MPa/min) (°C.) (min) 19 2.449 57 13.0 1800 10 20 3.027 57 13.0 2100 10 21 4.180 5713.0 1800 20 22 4.210 30 6.0 1800 10 23 4.417 30 6.0 1800 20

The SPS-sintered pellets formed in Example 2 were about 10 mm thick andhad bulk densities between 1.3 g/cm³ and 1.5 g/cm³. To illustrate thestrength of these samples, sample #20 was planned to be sintered 2100°C., but at about 1900° C., the die broke. The ram traveledsignificantly, crushing the graphite die. After the test was completed,the die was broken away from the sample. The sample remained visiblyintact, though slightly thinner than expected. This would indicate thatthe sintering occurs at temperatures less than 1900° C., that thestrength of SPS-sintered pellets is high, even at extreme temperatures,and that the sintered samples are strong enough to resist an appliedforce without fracturing.

The bulk densities of the samples with the graphite foil still attachedwere determined. For the samples weighing about 4 g (i.e., samples #21,#22, and #23), bulk densities were between 1.35 g/cm³ and 1.50 g/cm³.The volume resistivity and electrical conductivity of the samples werealso measured. These data are shown in Table 4. The samples are moreconductive than amorphous carbon, and nearly as conductive as graphite.

TABLE 4 Properties of samples prepared in Example 2: Electrical DensityResistance Resistivity Conductivity Sample (g/cm³) (Ω) (Ω · m) (S/m) 191.588 2.42 × 10⁻³ 4.98 × 10⁻⁵ 2.01 × 10⁻⁴ 20 1.715 2.02 × 10⁻³ 4.77 ×10⁻⁵ 2.10 × 10⁻⁴ 21 1.494 3.24 × 10⁻³ 1.23 × 10⁻⁴ 8.14 × 10⁻³ 22 1.3503.80 × 10⁻³ 1.62 × 10⁻⁴ 6.19 × 10⁻³ 23 1.429  3.7 × 10⁻³ 1.57 × 10⁻⁴6.37 × 10⁻³

Although the foregoing description contains many specifics, these arenot to be construed as limiting the scope of the present invention, butmerely as providing certain embodiments. Similarly, other embodiments ofthe invention may be devised which do not depart from the scope of thepresent invention. For example, features described herein with referenceto one embodiment also may be provided in others of the embodimentsdescribed herein. The scope of the invention is, therefore, indicatedand limited only by the appended claims and their legal equivalents,rather than by the foregoing description. All additions, deletions, andmodifications to the invention, as disclosed herein, which fall withinthe meaning and scope of the claims, are encompassed by the presentinvention.

What is claimed is:
 1. A sintered solid carbon product comprising ametal and a plurality of carbon nanotubes, wherein at least some of theplurality of carbon nanotubes are covalently bonded to other carbonnanotubes of the plurality.
 2. The sintered solid carbon product ofclaim 1, wherein the carbon nanotubes define a plurality of voidstherebetween having a median minimum dimension of less than about 100nm.
 3. The sintered solid carbon product of claim 1, wherein thesintered solid carbon product has a bulk density of greater thanapproximately 1.3 g/cm³.
 4. The sintered solid carbon product of claim1, wherein the sintered solid carbon product comprises aprojectile-resistant material.
 5. The sintered solid carbon product ofclaim 1, wherein the sintered solid carbon product comprises anelectrically conductive material.
 6. The sintered solid carbon productof claim 1, wherein the sintered solid carbon product has an electricalconductivity of at least approximately 1×10⁵ S/m.
 7. The sintered solidcarbon product of claim 1, wherein the sintered solid carbon product hasan electrical conductivity of at least approximately 1×10⁷ S/m.
 8. Thesintered solid carbon product of claim 1, wherein the sintered solidcarbon product comprises an electrode.
 9. The sintered solid carbonproduct of claim 1, wherein the metal comprises a catalyst residualselected from the group consisting of the elements of Groups 5 through10 of the periodic table.
 10. The sintered solid carbon product of claim1, wherein at least some of the carbon nanotubes contain a metal withingrowth tips of the carbon nanotubes.
 11. The sintered solid carbonproduct of claim 1, wherein the sintered solid carbon product has a bulkdensity of less than approximately 2.2 g/cm³.
 12. The sintered solidcarbon product of claim 1, further comprising at least one materialselected from the group consisting of a ceramic and a lubricant, whereinthe at least one material is interspersed in a continuous matrixsurrounding and in contact with the plurality of carbon nanotubes. 13.The sintered solid carbon product of claim 1, wherein the sintered solidcarbon product comprises a structural member.
 14. The sintered solidcarbon product of claim 1, wherein the sintered solid carbon product hasa thermal conductivity of at least approximately 400 W/m·K.
 15. Thesintered solid carbon product of claim 1, wherein the sintered solidcarbon product has a thermal conductivity of at least approximately 2000W/m·K.
 16. The sintered solid carbon product of claim 1, wherein thesintered solid carbon product has a thermal conductivity of at leastapproximately 4000 W/m·K.
 17. A sintered solid carbon product comprisinga metal and a plurality of carbon nanostructures, wherein the carbonnanostructures comprise carbon atoms bonded into a hexagonal lattice,wherein at least some of the plurality of carbon nanostructures arecovalently bonded to other carbon nanostructures of the plurality. 18.The sintered solid carbon products of claim 17, wherein at least some ofthe plurality of carbon nanostructures comprise multi-wall carbonnanotubes.
 19. The sintered solid carbon product of claim 17, wherein atleast some of the plurality of carbon nanostructures comprise nanobudsattached to carbon nanotubes.
 20. The sintered solid carbon products ofclaim 17, wherein at least some of the plurality of carbonnanostructures comprise single-wall carbon nanotubes.